The present invention relates to a controlled, spontaneous reactor system which can be used for successfully treating coals and other solid fuels to make those fuels competitive with lower moisture, higher Btu rated coals. This invention is applicable not only to U.S. coals, but coals from Canada and throughout the world.
Subbituminous and lignitic coals typically found in the Western portions of the United States have higher moisture contents than Eastern bituminous coals. This results in lower rated efficiency, and in some cases, increased polluting effect. As a result, by way of example only, boilers designed and rated for certain performances will be derated or perform at lower levels of efficiency when fed high moisture, low-rank coal. This problem of derating substantially reduces a potentially available market for Western low-rank coals.
It is generally known in the art that some of the problems for low-rank coals can be overcome by treating such coals in fluidized bed reactors. Thus, for example, drying of low-rank coals in fluidized bed reactors prior to shipment in order to have a decreased freight rate, is known. So too is drying in an effort to increase overall boiler system efficiency. However, in the past such dryers have been built with single special uses in mind, and require specific drying processes, temperatures, conditions, dryer geometry, coal sizing, etc., all depending upon the nature of the use and the overall objective of the treatment process. These types of dryers have the disadvantage of producing a high degree of fines and/or a product that will spontaneously combust uncontrollably upon storage or shipment. This requires special treatment of the coal prior to shipment. They also require coal sizing within tight tolerances to operate correctly. A relatively high gas temperature is also required for typical thermal dryers. These factors result in high operating costs and a large amount of coal fines that are difficult to use or sell.
It can therefore be seen that there is a real and continuing need for an overall treatment reactor that can provide a generalized method of treating coal to realize a reduction in overall capital and operating costs, limit the amount of presizing of the coal, to increase boiler system efficiency and overall efficiency, and to reduce emission levels for steam or power production.
Passage of the Clean Air Act Amendments of 1990 has drastically changed the strategies that a utility must use to comply with federal emission standards. A number of different technologies have been developed for NO.sub.x control in cyclone boilers; including selective catalytic and noncatalytic reduction (SCR and SNCR), reburning, and slagging combustors. SCR, SNCR, and slagging combustors are expensive to install. Reburning appears to be the most attractive NO.sub.x control method. Reburning using natural gas as the reburn fuel has been demonstrated at Ohio Edison' Niles plant, while micronized coal has been demonstrated as a reburn fuel by Wisconsin P&L in their Nelson Dewey plant. Although both of these plants demonstrated that reburn was a successful method of reducing NO.sub.x emissions in a cyclone unit, these methods are not preferred by most utilities due either to the unavailability of gas at the plant site, or the inherent safety problems associated with handling the micronized coal that is used for reburning. Therefore, there is a need for another inexpensive source of reburn fuel that can be used to control NO.sub.x in cyclone boilers.
Another objective of the present invention is to provide a controlled, spontaneous, fluidized bed reactor, which can be operated as required to eliminate derates, to reduce NO.sub.x and fouling for low-rank coals used in cyclone boilers, for generalized coal drying, to extend the load control range and to reduce NO.sub.x and N.sub.2 O emissions for fluidized bed combustion, and for char production from coal.
The method and manner of accomplishing each of the above objectives will become apparent from the detailed description of the invention which follows.